A recent development in memory devices involves spin electronics, which combines principles of semiconductor technology and magnetism. The electron spin, rather than the charge, may be used to indicate the presence of a “1” or “0” binary state in a magnetic tunnel junction device. One such spin electronics device is a magnetic random access memory (MRAM) device. FIG. 1 illustrates a simplified schematic for a portion of a typical MRAM device 20. In an MRAM device 20, conductive lines 22 (i.e., word lines and bit lines) may be positioned perpendicular to each other in different metal layers. The conductive lines 22 sandwich a magnetic tunnel junction (MTJ) 30. Each MTJ 30 includes at least two magnetic layers 31, 32 separated by a tunnel barrier layer 34 between them. The storage mechanism relies on the relative orientation of the magnetization of the two magnetic layers 31, 32, and the ability to discern or sense this orientation electrically through electrodes (i.e., the conductive lines 22) attached to these magnetic layers 31, 32. Hence, digital information represented as a “0” or “1” is storable in the relative alignment of magnetic moments in each MTJ 30. For general background regarding MTJ devices and MRAM devices, reference may be made to U.S. Pat. Nos. 6,538,919, 6,385,082, 5,650,958, and/or 5,640,343, for example. Each of these patents is incorporated herein by reference.
In a magnetic tunnel junction device 20, it is essential that the two magnetic layers 31, 32 in each MTJ 30 are isolated from each other by the tunnel barrier layer 34. Although shown as single layers for purposes of simplifying the illustration, the magnetic layers 31, 32 are typically each formed of multiple stacked layers of various materials. FIGS. 2 and 3 illustrate a typical process for forming a MTJ 30 for a magnetic tunnel junction device 40 (e.g., an MRAM device). FIG. 2 is a cross-section view showing unpatterned magnetic tunnel junction layers 29 formed over an underlying layer, which includes a conducting line 22 (e.g., a word line or a bit line) formed in an insulating layer 44. The magnetic tunnel junction layers 29 include two magnetic layers 31, 32 with a tunnel barrier layer 34 sandwiched there between. A hard mask 42 is located atop the upper magnetic layer 31. At this stage, the hard mask 42 has already been etched and patterned. Next in this conventional process, both magnetic layers 31, 32, along with the tunnel barrier layer 34, are etched in alignment with the hard mask 42 using an etching process, such as wet etching, reactive ion etching (RIE), or ion milling, for example. RIE is preferred for its ability to anisotropically etch in a controlled direction (e.g., to provide vertical sidewalls for the MTJ 30). FIG. 3 shows the MTJ 30 formed from such process. Note that a portion of the hard mask 42 may remain after this step, as shown in FIG. 3, and any remaining hard mask 42 may be later removed, if so desired or needed.
Although RIE and ion milling provide the advantage of anisotropic (directional) removal of material, the main drawback of RIE and ion milling is the discharge of displaced particles being removed during the process, which can be projected in many different directions. Hence, a major concern and problem with the above-described process of forming the MTJ 30 (see FIGS. 2 and 3) is re-deposition of resputtered conductive material from the magnetic layers 31, 32 and/or the underlying conductive line 22 onto the MTJ 30 at the tunnel barrier layer 34. Such re-deposition may cause a short between the two magnetic layers 31, 32, which need to be electrically insulated from each other across the tunnel barrier layer 34 for the MTJ 30 to work properly. Thus, there is a need for a method to form the MTJ 30 while significantly decreasing or eliminating the risk that electrically conductive materials may be re-deposited onto the MTJ 30 causing a short. Another problem is that re-deposited conductive material may form a bridge between conducting lines, which should be avoided as well.
One method of forming the MTJ 30 that has been tried with the intent of avoiding re-deposition of conductive material onto the MTJ 30 and elsewhere is using a high temperature plasma environment (e.g., 300–400° C.). This method is intended to volatilize the displaced particles so that they are more easily removed from the reaction chamber to avoid re-deposition. A major drawback of this method, however, is the high temperature stress experienced by the device. Such high temperature stress can damage the device and/or negatively affect its performance abilities. Thus, it would be highly desirable to provide a method of forming the MTJ 30 under lower temperature conditions.
Other problems encountered using prior methods of forming an MTJ 30 include corrosion of copper lines and low etch selectivity for TiN hard masks, which are commonly used. Hence, a need exists for a method of forming an MTJ 30 while reducing or preventing corrosion to underlying conducting lines 22 and/or while providing better etch selectivity for the hard mask 42.